Red: *** Error: word has no value! when calling external library function

Question

I am following the example from the Red documentation here: http://static.red-lang.org/red-system-specs.html#section-14

This is my code:

Red []

#import [
   "SDL2.dll" cdecl [
       sdl_init: "SDL_Init" [
           flags [integer!]
           return: [integer!]
       ]
   ]
]

rc: sdl_init 0

When I execute this with the Red interpreter, I get the following error on the last line:

*** Error: word has no value!
*** Error: word has no value!

Compiling with red -c gives a more useful error:

*** Compilation Error: undefined word sdl_init
*** in file: %/C/temp/red/sdl.red
*** near: [sdl_init 0]

Obviously the sdl_init definied during the import of the library is out of scope by the time it gets to the last line.

How do you explain the example in the documentation then? If I initialize sdl_init to some arbitrary value prior to the import, it still retains the same value after the #import block.

Answer 1

Remember, you are reading the Red/System (The C-like language) spec, not the Red spec, so the #import must be done in Red/System. A couple of notes:

1. If you want to reference Red/System within Red there are a couple ways:

Make a separate SDL2.reds script with the imports,

Red/System []
sdl: context [
    #import [
       "SDL2.dll" cdecl [
           sdl_init: "SDL_Init" [
               flags [integer!]
               return: [integer!]
           ]
       ]
    ]
]
rc: sdl_init 0

To use it in Red, use #include %SDL2.reds in the script, and you can make a routine

#include %SDL2.reds
initialize: routine ["SDL Initialize" ][
    with sdl [sdl_init 0]
]

Or your can use the directives: #system-global [] or #system [] - I don't know that this is a good idea for using #import, but you can call the SDL2.reds functions in it.

#include %SDL2.reds
#system [with sdl [sdl_init 0]]

2. There is an SDL1 binding that already exists here. There are dependencies from other bindings as well though. I have a mirror of all those fossil bindings (updated a couple days ago).

Answer 2

The document you refer to is the Red/System specification and does not cover Red. You have actually written a Red/System program. (Apart from having used a Red[] header). It should perform as you expect, if you compile and run it.

If you want to access an external library from Red you need to use a routine!, a Red/System function that is called from Red. If you are only passing integer values between Red and the external function it is quite straight forward as integer values are automatically "marshalled" between Red and Red/System. (This is not the case with other datatypes).

A Red program that includes a routjne! needs to be compiled and cannot be run from the Red console at this stage. Once the Red compiler is self-hosted (Red 2.0), there should be a just-in-time compiler so you will be able to run such code in the Red console.